Far Infrared Absorption in Superconducting Lead Alloys

Abstract

Measurements have been made at 1.3\ifmmode^\circ\else\textdegree\fi{}K of the absorption of far infrared radiation in the surface of bulk samples of superconducting alloys composed of lead with various amounts of thallium, bismuth, or tin. The data provide the first direct experimental evidence for a sharp gap edge in superconducting alloys. The width of the energy gap decreases as thallium or tin is added to lead, and increases as bismuth is added. For concentrations up to 7.7 at.% thallium the fractional decrease in the gap width is approximately proportional to the fractional decrease in the critical temperature, but is significantly greater than the latter for 10.0 at.% thallium. The steepness of the absorption edge is roughly the same in all of the samples, and is greater than might have been expected from theoretical calculations, although these have only been performed in the exteme anomalous limit, which none of our samples strictly satisfy. The structure which was previously observed on the absorption curves of pure lead and mercury is also present in all of our alloys, indicating that it is probably not a result of anisotropy.